Had been searching for a beige Zenith Royal 475 to accompany my black set and recently acquired this one as part of a trade with another ARF member. It was in relatively good cosmetic shape other than a few chips on rear cover and holes drilled in side by a previous owner for an antenna connector. One of the plastic chassis mounting studs had broken off and was missing as was its hardware. It was also missing the emblem… But it worked great with original electrolytic caps !

Completely disassembled and cleaned/detailed---tuning dial came loose during cleaning process and had to be reattached using Permatex Ultra-Black RTV… Gently removed trace chrome residue from volume knob with stainless-steel brush and 91% isopropyl alcohol, then polished center with Novus-2. The volume knob was “tight” to turn and discovered that someone had replaced the original control with one having a larger shaft which they’d filed down to accommodate original knob. They had drilled out hole in front panel, but not enough and shaft was binding---so enlarged it and all fine… The missing mounting stud was more of a challenge. Decided that a 10-32 x 3/8” Phillips pan-head screw’s head could be filed down to fit within the circular “cup” which supported edge of chassis and surrounded the “missing” post which had broken off cleanly from panel. Attached screw to large fender washer and placed a smaller washer of required thickness and ID of screw head to ensure that finished end would be “square” and filed until this washer fully contacted tool surface. Then “chucked” the screw into drill and removed very small amounts at a time from circumference with file until it fit snugly within that plastic “cup”. Cleaned all contacting surfaces thoroughly with alcohol, then applied gel Superglue & pressed in. It worked perfectly & looks as if it belonged---chassis now secure and sturdy…

Reassembled, installed one of my laminated emblems ( thanks again to Patrick for his help with these ), installed batteries, and turned it on… Worked & sounded great with dial tracking just a bit off, but after awhile, sound became distorted and I pulled cover to investigate---detected a “hot” odor and one of the P-P output transistors was almost too hot to touch ! Arghhh ( double Arghhh ) !!! So out came the batteries and back on the shelf until time permits troubleshooting---any ideas or experiences with this type of “sudden” failure ???

Also hope to find a junk beige set with a good, chip-free back cover...

Below are several pics including “fabricated” chassis mount stud--two more "angle shots" are in following post ... John

Attachments:

ARF 475 PAIR.jpg [ 89.1 KiB | Viewed 4104 times ]

ARF 475 BK CMP.jpg [ 59.91 KiB | Viewed 4104 times ]

ARF 475 FRT CMP.jpg [ 155.83 KiB | Viewed 4104 times ]

ARF 475 INS.jpg [ 129.79 KiB | Viewed 4104 times ]

ARF 475 CHAS INST STD.jpg [ 98.63 KiB | Viewed 4104 times ]

ARF 475 MOUNTING STUD.jpg [ 118.34 KiB | Viewed 4104 times ]

ARF 475 PRTS IN.jpg [ 121.65 KiB | Viewed 4104 times ]

ARF 475 PRTS OUT.jpg [ 127.79 KiB | Viewed 4104 times ]

Last edited by xrhonda91 on Feb Mon 05, 2018 2:24 am, edited 3 times in total.

Well, checked the P-P output transistors Friday and found that one was "leaky"... Also checked & took measurements of other components in audio out circuit and it appeared okay.

Realize that these need to be a matched pair and was thinking about trying a set of Japanese transistors pulled from a junk low-power audio-amp since I don't have extras of that Zenith P/N... Guess I should get online and try to find the specs for comparison first...

You could just try a pair of outputs from another set and see if you like the way it sounds. I don't really know what all this 'matched outputs' from manufacturers is all about other than maybe battery life. We are not talking about a Marrantz.

Well, a bad radio issue had a "measure" of positive outcome ! Recently bought another ebony Royal 475 which turned out to have been partially submerged in water for a long period of time and was severely rusted internally---but after sending pic to seller, he refunded most of cost and I was able to salvage the audio output transistors from it. After careful cleaning, installed them in this radio and it now sounds excellent with what seems to be twice the volume & sound quality that it had even before it became distorted---Very thankful !

Was having some odd, intermittent "issues" with this radio and decided to recap it... All caps seemed okay except the EIA ceramic dual 100 uF electrolytic which was cracked on the bottom ( non-visible ) side and was completely open---which from my & others' experiences is very common... Replaced this and all other caps and it seemed fine for awhile---then turned it on tonight and got a lot of "crashing" noise ( new problem ! ) as though something was shorting. Tapping on case made it much worse. Turned off, then on again and now it is fine---so treating it gently until I have more time to troubleshoot ! Was very careful "dressing" & inspecting the new cap leads as well as reassembling, so have no idea what's going on now...

Am wondering now if the original P-P output transistors were degraded by "spikes" from turning set on and off with a possibly bad switch ( original control had been replaced by a previous owner ) since both sections of that filter/bypass cap were open ? A large electrolytic across power source ( at switch output ) would "dampen" such events if it were good... Hmmm...

After thinking about it awhile, remembered that the negative terminal on the battery holder didn't solder very well and possibly the connection became loose while it was handled during disassembly and recapping the other day. That will be the first thing looked at when time permits...

My ebony set had same problem and that terminal was so bad that solder wouldn't adhere at all---even after cleaning it with Scotch-Brite. Put a 9V connector in it temporarily and may do same with this one. Guess a permanent fix would be a very small screw that would fit the terminal hole along with lockwasher, nut, and lug for wire...

Glad to say that the intermittent noise/reception issues have been corrected !

Was using this set for voltage comparisons to troubleshoot the "flood victim" AVC problem and while inside, "wiggled" the negative battery wire---no problem... Then tapped on PCB and all sort of noise appeared intermittently. Gently tapped individual components and found that both the RF & mixer transistors were causing noise of differing types---simply removed & reinstalled each one into their sockets and all fine ! No visible sign of dirt or corrosion on leads or in sockets, but that was all it took to fix it...

How many have pulled a radio off of shelf after it has sat for years following "refurbishment" and it failed to work ?

Was going through some favorite sets this morning and this one decided to fail in multiple ways (again) ! At first it did nothing---then played at full volume after "wiggling" volume control. Tapped on PCB and volume control decided to work for awhile, but it also had another noisy intermittent which turned out to be a transistor socket connection. After all this, performance wasn't quite right, so tweaked IFT's to bring it back (hmmm---internal cap(s) failing ?). Then volume control failed again, so after determining that wiring and PCB traces were okay, checked pot and found that the connections at terminal crimps were erratic---tried soldering and killed it entirely... This was a very cheaply made control which had been installed by a previous owner who had filed the shaft down to "somewhat" fit the knob and drilled out hole in front panel to accommodate the larger diameter beneath the modified portion.

Still had a few new split-splined shaft 5K controls in parts drawer, so used one as a replacement. Wanted to come up with an easy, but effective way of "mating" the knob to this shaft without further modification. Noticed the slot on plastic knob shaft was 1/4" wide and thought a small piece of steel would make a good "coupler"---Found a steel bracket that was proper width and thickness in junk bin and cut off a short section. Filed the cut end smooth and it fit perfectly within the knob slot. It was a bit loose within the split aluminum control shaft, so put a couple thicknesses of clear packing tape over end to make it snug. It works great !

Another (unexpected) revisit complete----wonder how it will do next time it's tried ??? Am listening to big-band Sunday night on Zoomer 740 right now to give it some "exercise"...

Project pics below---maybe this idea will help others with control issues ?

How many have pulled a radio off of shelf after it has sat for years following "refurbishment" and it failed to work ? .....

Yes John. This one right here. A Hoffman KP-709X. It's a fantastic little radio.... most of the time. I turn it on at least once or twice a year just to see if it still works, but every now and then... nothing! The audio is working as the background hiss increases with the volume control. I know the oscillator is running as it can be heard on an adjacent radio, and at the right frequency too. If I open it up and so much as touch a probe to any part of the RF or IF chain it begins to work, and it will keep on working for months and months then the cycle repeats. I've been through this at least 10 times. I've even swapped out the mixer and IF transistors from a parts radio, but the problem still persists. I really don't know how or if I will ever find the problem.

Hey Matt---I've had at least one radio with a similar issue where simply touching a point with a probe would restore operation. Am wondering if perhaps a bias resistor has drifted high and transistor is simply at the threshold of "turning on" and the noise generated by probe contact provides the necessary stimulus---and it keeps going until something causes it to again drop just below that point ??? Can't even remember which radio(s) had this problem, but if I run across one, will go that route in troubleshooting...

Thanks John. I was following up on your lead when I discovered that a resistor was missing! R5 is a 250K resistor shown on the board layout (mounted and soldered on the trace side of the board) and on the schematic, connected between the collector of X1 and pin 2 of A3. In fact pin 2 of A3 was just hanging in the breeze! It simply pokes through the PC board without even the luxury of a solder pad. I could see no evidence of anything ever being soldered to it.

I can't say just yet that this solved the problem, but my guess is that it did.

.... Are the "X"'s on the SAMS or did you add them ? I've seen that done on other schematics along with a note stating reason...

John -- I did not add the "X"'s and there is no note since the resistors are not included in the parts list. However, the Sam's for the Hoffman EP706 shows a capacitor, C16 with the "X"'s. The note for C16 in the parts list says "Not used in some versions." So this radio probably never had an R5. I'm not sure if this fixed it, but the only positive here is that touching the collector of X1 or any pin of A3 with a probe is usually what gets it going.